Magnetic resonance imaging (MRI) has provided dramatic new capabilities for diagnostic medicine. MRI enables the acquisition of high resolution, three-dimensional images in the detection of a wide variety of physical abnormalities, and recent advances in dynamic MRI are providing real-time imaging. Over 30% of MRI scans are now acquired using a paramagnetic contrast agent, which enhances the proton relaxation and hence image quality. Gadolinium complexes are most widely used, and these complexes currently are all based on a poly(amino-carboxylate) ligand scaffold . While effective, the relaxivity values (3 - 5mM"V1) of these agents are only a few percent of that theoretically possible, requiring gram amounts of Gd per administration. Attachment of the agent to macromolecules increases the relaxivity by lowering the rotational correlation time, but the rate of water exchange from the gadolinium center then becomes the limiting factor. This project has developed gadolinium complexes (based on a hexadentate hydroxypyridonate ligand scaffold) that are stable and have substantially higher relaxivity due to a water exchange rate at least two orders of magnitude higher than commercial agents. Having developed the agents and demonstrated their stability and fast rates of water exchange, it is now proposed to continue their development for enabling new kinds of imaging. Relaxivities of more than 100mM~1s"1 are the target. This would require much less Gd for images and, more significantly, enable new types of imaging with MRI. Aims include the development of stable agents with 3 (as opposed to the current 1) coordinated water molecules, thus tripling the relaxivity; novel designs of supramolecular Gd clusters;agents that dock to specific targeted biomolecules;and full characterization of those thermodynamic and kinetic properties of these complexes in aqueous solution that are related to their MRI enhancement. The goal is a second generation of MRI agents that utilize relaxivities of up to two orders of magnitude greater than the clinical agents in use today.